1. Field of the Invention
The present invention relates to an improvement in an optical system of a photoscanner, and particularly, in an optical system having a fall-correction function for a rotary mirror or the like, and an optical system arrangement which can make the effective radius of curvature of lenses used large and which can prevent generation of a ghost image.
2. Description of the Prior Art
Photowriting apparatus for scanning with a laser beam on a photosensitive body are known to use a photodeflector such as a rotary polyhedral mirror, a hologram scanner and the like, an image forming element such as an f.theta. lens and the like, and a correction optical system for correcting so-called face-fall errors resulting from irregularities of the deflecting elements such as a fall error of each mirror face of the rotary polyhedral mirror, a deviation in position or eccentricity of a hologram of a hologram scanner.
FIG. 1 shows one example of a well known optical system as described above. A rotary polyhedral mirror is used as a photodeflector (light source not shown).
A gas laser or a semiconductor laser is used as a light source as is known. In case of a gas laser, a laser beam emitted from the light source is modulated by a modulator such as AOM and formed into a suitable beam diameter by a beam expander or the like. On the other hand, in case of a semiconductor laser, a laser beam modulated and emitted is formed into a suitable beam diameter by a coupling optical system or a shaping optical system. These beams are incident upon a rotary polyhedral mirror 1 by a first anamorphic optical system, in the form of an elongated elliptic beam having a long axis nearly perpendicular to the rotary axis thereof.
A laser light flux deflected by the rotation of the rotary polyhedral mirror 1 is formed on a photosensitive body 5 by means of an f.theta. lens 2 and is subjected to uniform scanning.
In the aforesaid optical system having a face-fall correcting function, as shown in a light path view of FIG. 2 in a so-called sub-scanning direction perpendicular to the scanning direction of the aforesaid laser beam, there is arranged a second cylindrical lens 4, and the rotary polyhedral mirror 1 and photosensitive body 5 are in a geometrically conjugated relation with the cylindrical lens 4 and f.theta. lens 2. With this, even if the laser beam should be deviated as indicated by the dotted lines by the face-fall of the mirror face 1, the spot position on the photosensitive body is kept constant to prevent occurrence of irregularity in the pitch of a scanning line.
In the drawing, a reference numeral 3 designates a plane mirror which is provided to refract the light path so as to permit the laser beam to be incident upon from the top of the photosensitive body 5.
In such a laser optical system, in practical use by means of a laser printer or the like, there is a tendency that lenses of short focal length are often used for miniaturization of the devices. Especially, cylindrical lenses used in the scanning optical system are often flat convex and flat concave lenses due to the limitation in manufacture. In addition, a material having low refractive index is generally used, and therefore, the radius of curvature used tends to be smaller and the volume productivity of manufacture is harder to obtain. For example, for lengthy cylindrical lenses having a length of from 200 to 300 mm and a width of from 15 to 20 mm, four lenses at the most are obtained by simultaneous manufacture when the radius of curvature is 20 mm, while if the radius of curvature is 40 mm, the number of lenses which can be obtained by simultaneous manufacture increases to about three times thereof.
Moreover, in the optical arrangement as described above, dust and toner entered are accumulated on the second cylindrical lens, not only resulting in the lowering of permeability but also the generation of white streaks caused by the accumulated matter on the copy in a sub-scanning direction due to the uneven accumulation. In addition, in this arrangement, the reflected light from the surface of the second cylindrical lens 4 and the scattered reflected light from the surface of the photosensitive body 5 are returned to the rotary polyhedral mirror 1 and reflected again to generate white streaks which result in a stationary ghost image on the photosensitive body 5.